1. Field of the Invention
The present invention relates to an aluminum nitride (AlN) dispersed powder aluminum alloy, and more particularly, it relates to an aluminum nitride dispersed powder aluminum alloy that is lightweight, high in wear resistance, seizure resistance, heat resistance and thermal properties and that has excellent toughness and machinability, and to a method of preparing the same. Such an alloy is applicable to compressor parts such as a vane and a rotor, sliding parts such as an oil pump rotor and a shoe, engine parts such as a valve lifter, a retainer, a cylinder liner and a connecting rod, and a heat sink.
2. Description of the Prior Art
A generally known wear-resistant powder aluminum alloy is prepared by mixing and adding hard grains or fibers of alumina (Al.sub.2 O.sub.3), silicon carbide (SiC) or aluminum nitride (AlN), for example, into an aluminum alloy powder forming the base, in order to improve its wear resistance, conformability to a counter material and counter attackability. However, such hard grains or fibers come loose and fall out from the base during sliding and thereby form an abrasion powder, which disadvantageously induces abrasion damage or seizure to reduce the wear resistance. Namely, the hard grains simply added to the base fall out during sliding to induce seizure or abrasion. In preparation of the wear-resistant powder aluminum alloy, further, the added hard grains having fine grain diameters of about 3 to 10 .mu.m segregate or aggregate to reduce mechanical properties or wear resistance of a resulting sintered body. In order to solve this problem, the mixing step must be repeatedly carried out. In addition, the employment of high-priced hard grains leads to an economic problem.
In order to suppress the problem of hard grains falling out of the base during sliding, methods of dispersing hard grains in aluminum alloys without simply adding the grains to the base have been studied. Such methods include a method of heating a raw material powder mainly composed of aluminum (Al) in a nitrogen gas atmosphere for continuously forming and dispersing AlN having excellent slidability on old or prior grain boundaries or on old or prior grain surfaces by direct reaction between nitrogen gas (N) and Al. For example, Japanese Patent laying-Open No. 6-57363 (1994) "Nitrogen Compound Aluminum Sintered Alloy and Method of Preparing the Same" or Japanese Patent Laying-Open No. 6-33164 (1994) "Method of Preparing Nitride Dispersed Al Alloy Member" disclose such a method. According to this method, the AlN coating layers are homogeneously formed and dispersed on all old or prior grain boundaries or on old or prior grain surfaces forming the base for a powder aluminum alloy, whereby a powder aluminum alloy having excellent wear resistance and seizure resistance can be prepared.
In such a powder aluminum alloy prepared by forming and dispersing AlN coating layers by nitriding, however, the nitriding takes place continuously and substantially uniformly on all grain surfaces of the aluminum alloy as described above, and hence the resulting AlN coating layers exist continuously on all prior grain boundaries or prior grain surfaces in a sintered body. Consequently, the AlN coating layers inhibit the metallic diffusion bonding ability between the prior grains, and thus remarkably reduce the toughness of the material, such as the elongation or the impact value. When the powder aluminum alloy is worked into a component, weak bonding between the grains results in a problem in machinability, such as chipping on an end portion of a sample. In addition, remarkable plastic deformation must be applied in order to part the AlN coating layers that have been continuously formed in the aluminum alloy, leading to a remarkable restriction on the possible shape of the component.